Planar lightwave circuits (PLCs) in telecommunications, such as arrayed waveguide gratings (AWGs) used to multiplex or demultiplex multiple optical signals transmitted over a single optical fiber, typically are formed of birefringent materials such as doped SiO2 or LiNbO3. Birefringence in planar waveguides complicates the design and operation of PLCs. Planar waveguides as are found in AWGs typically have different propagation constants for TE (transverse electric) and TM (transverse magnetic) waveguide modes caused by stress-induced birefringence introduced into the waveguides due to mismatched coefficients of thermal expansion of the substrate, core, and cladding. For AWGs, this difference in propagation constants produces a wavelength shift between each AWG channel's peak response to the TE input polarization and that channel's response to the TM polarization. The difference between the wavelength of peak TE transmission and the wavelength of peak TM transmission is called a polarization dependent wavelength shift (PDW). Even a small PDW may be a concern because of the problems it may cause, such as poor channel isolation as well as increased polarization-dependent loss (PDL).
One way to compensate for birefringence is to formulate a waveplate of a rigid polyimide polymer having a heat resistance of 300° C. or higher, as disclosed in U.S. Pat. No. 6,115,514. The waveplate changes the polarization state of light passing through the waveplate. The patent expresses a clear preference that the waveplate is formed of a rigid polymer with no more than 2 rotatable chemical bonds. The interchain orientation and intermolecular interactions of the polymer backbones creates birefringence and thermal stability as a sheet of the polymer is uniaxially drawn or stretched. Further, the patent states that it is necessary for the polymer to have a heat resistance in excess of 300° C.
Another way to compensate for birefringence is to use a birefringent crystal waveplate. Reported in 1992, a quartz waveplate was used to correct for polarization dependence in an arrayed-waveguide grating (AWG) multiplexer, although the method had high loss. More recently, a thin (10 μm thick) LiNbO3 half waveplate created using crystal ion slicing was demonstrated that could substantially reduce loss. In this paper, concerns are expressed about the hygroscopic nature of polyimides and possible long-term changes due to environmental factors. In addition, the LiNbO3 work stated the desire to improve polarization mode conversion ratios and the desire to create thinner waveplates for insertion loss reduction.